1. Field of the Invention
The present invention relates to a process for concurrently fractionating and hydrotreating a full range naphtha stream. More particularly the full boiling range naphtha stream is subjected to simultaneous thioetherification and splitting into a light boiling range naphtha, a medium boiling range naphtha and a heavy boiling range naphtha. Each boiling range naphtha is treated separately to achieve a combined desired total sulfur content. More particularly the medium boiling range fraction, containing thiophenic compounds, is subjected to higher severity treatment to selectively remove the sulfur compounds.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the xe2x80x9csweetxe2x80x9d (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:
(1) RSH+H2xe2x86x92RH+H2S
(2) RCl+H2xe2x86x92RH+HCl
(3) 2RN+4H2xe2x86x922RH+2NH3
(4) ROOH+2H2xe2x86x92RH+2H2O
Typical operating conditions for the HDS reactions are:
After the hydrotreating is complete the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common solution has been prefractionation prior to hydrotreating. The prefracfionation produces a light boiling range naphtha which boils in the range of C5 to about 250xc2x0 F. and a heavy boiling range naphtha which boils in the range of from about 250-475xc2x0 F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
In addition to treating the lighter portion of the naphtha to remove the mercaptans it traditionally has been used as feed to a catalytic reforming unit to increase the octane number if necessary. Also the lighter fraction may be subjected to further separation to remove the valuable C5 olefins (amylenes) which are useful in preparing ethers.
More recently a new technology has allowed for the simultaneous treatment and fractionation of petroleum products, including naphtha, especially fluid catalytically cracked naphtha (FCC naphtha). See, for example, commonly owned U.S. Pat. Nos. 5,510,568; 5,597,476; 5,779,883; 5,807,477 and 6,083,378.
In a common embodiment a full boiling range FCC naphtha is fed to a naphtha splitter which contains a thioetherification catalyst in the upper portion of the splitter. Hydrogen is fed to maintain the catalyst in the hydride state. The mercaptans in the light fraction react with the diolefins contained therein (thioetherification) to produce higher boiling sulfides which are removed as bottoms along with the heavy (higher boiling) FCC naphtha. The bottoms are then fed to a second distillation column reactor containing a standard hydrodesulfurization catalyst where the remaining sulfur compounds (including the sulfides produced in the splitter) are converted to H2S which can be separated as a vapor. Finally the overheads and bottoms from the second column are combined and passed through a polishing reactor to produce a naphtha product with the desired sulfur content of about 50 wppm. In this way the lower boiling olefin containing fraction is not subjected to conditions conducive to the hydrogenation of the olefins contained therein.
It has now been found that the light FCC naphtha cut just below the light fraction also contains mercaptans and a significant amount of thiophenes. The mercaptans in this cut may be removed by the thioetherification, but the thiophenes remain and this cut will not meet sulfur requirements.
It is an advantage of the present invention that the sulfur may be removed from the light olefin portion of the stream to a heavier portion of the stream without any substantial loss of olefins. Substantially all of the sulfur in the heavier portion is converted to H2S by hydrodesulfurization and easily distilled away from the hydrocarbons. Also the sulfur in the middle cut will be lowered to specification.
Briefly the present invention is process for removal of sulfur from a full boiling range fluid cracked naphtha stream to meet higher standards for sulfur removal, by splitting the light portion of the stream utilizing a three way naphtha splitter as a distillation column reactor to treat the lightest boiling range naphtha to remove the mercaptans contained therein by thioetherification and treating the components of the naphtha feed with the process that preserves the olefinic while most expediently removing the sulfur compounds. Utilizing a three-way naphtha splitter as a first distillation column reactor to treat the lightest boiling range naphtha to remove the mercaptans contained therein by reaction with diolefins in the naphtha to form sulfides. A mid-cut sidedraw of a thiophene cut is taken from the first distillation column reactor which may be passed directly to a polishing reactor or more preferably fractionated in a second column depending on the constitution of the sidedraw, contacted with a catalyst in the presence of hydrogen to hydrogenate diolefins.
A side draw or thiophene cut is fed to a reactor for polishing reaction to reduce the sulfur content to that desired, i.e., about 50 wppm or less. The polishing reactor may be either a single pass downflow reactor or a catalytic distillation column reactor. In either mode, a low sulfur, low olefin heavy oil type material is fed along with the thiophene cut to maintain a liquid phase in the reactor. The heavy oil is recovered for recycle in the polishing reactor.
The advantages of this system are that the size and capital investment of the hydrodesulfurization distillation column reactor is reduced; the level of recombinant mercaptans coming from the hydrodesulfurization distillation column is reduced, and finally, there is a potential savings in octane due to the milder treatment of the olefin rich thiophene cut.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure.